The present invention relates to printer drive trains, and more particularly to a printer drive train for providing and maintaining ribbon tension upstream and downstream of a print head.
Many printers incorporate a ribbon used as a carrier or substrate for the print material (e.g., ink) that is transferred to a print media during the printing process. For example, thermal transfer printers include a thermal print head that selectively heats the ribbon to transfer ink onto a print media, such as a label. During a typical printing cycle, the ribbon is unwound from a supply spool, directed downstream between the thermal print head and a drive roller where it comes into contact with and prints to the print media, and is subsequently wound about a take-up spool.
To move the print media and ribbon upstream and downstream of the print head, a drive motor (e.g., a stepper motor) is engaged to a drive train that in turn is coupled via gears to the drive roller, supply spool, and/or take-up spool. This complex series of gears creates several challenges related to providing and maintaining the optimal tension in the ribbon both during and between printing cycles.
Improper tension in the ribbon may cause slack in the ribbon both upstream and downstream of the print head. A ribbon exhibiting excessive slack can degrade print quality and lead to other issues with the operation of the printer. For instance, if the tension of the ribbon drops below an operational threshold, creases or wrinkles may develop in the ribbon resulting in print defects. Moreover, slack ribbon is increasingly susceptible to thermal distortion resulting from the heat of the thermal print head and/or may result in drag on the print media resulting in visible scuff marks formed on the print media.
Another challenge arises between printing cycles in maintaining ribbon tension such that a subsequent printing cycle begins with a properly tensioned ribbon. This issue is exacerbated when the direction the ribbon cartridge is being driven is reversed (i.e., from downstream to upstream and vice versa). Moreover, backlash inherent in the gear train also presents a challenge to ensure that the ribbon is tensioned before the print cycle begins. Without the appropriate tension applied to the ribbon, excess ribbon slack may be introduced causing any of the issues discussed above.
Present designs incorporate tensioning elements within the ribbon cartridge to prevent freewheeling of the supply spool and take-up spool when not being driven by the drive motor. However, internal tensioning elements in the ribbon cartridge are less than ideal because of the added costs each element adds to the ultimately disposable ribbon cartridge.
In light of the above challenges, a need exists for a drive train that provides and maintains proper tensioning of a ribbon. In particular, a need exists for a drive train that provides and maintains sufficient, but not excessive, tension in multiple ribbon feed directions and properly coordinates with the rotation of the drive roller.